Vibration in built-up structures causes problems ranging from radiation of acoustic noise to damage to sensitive instrumentation. Built-up structures are any composite of beams, plates, joints and other structural components connected to form a nominal single unit. Examples of built-up structures are bridges, submarines and automobiles. All of these structures suffer from vibrations to one degree or another. One kind of built-up structure is the space frame or truss. Space frames or trusses are used in a variety of applications from off-shore structures to bridges to antenna platforms. Trusses are composed of beam members, which may be closed, as for example box or tubular beams, or open, as for example I-beams. In either case, structural vibration of the base structure is caused most commonly by external forcing from attached equipment or fluid flow. In many structural applications it is desirable to reduce the level of such vibrations. In the context of a submarine, to cite a single example, the desire to minimize vibrations stems from the need to reduce acoustic radiation into the water around the submarine so as to reduce the probability of detection.
The use of granular material to effect structural damping is well known. Sand and lead shot have been used for some time to provide such damping. For example, sand is used to fill the hollow spaces of tubular beams. Sand and lead shot, though effective in providing damping primarily through a mass loading effect, are very heavy so that their use extracts a high weight penalty. For example, sand has a bulk specific gravity of not less than 1.5 and lead shot has a bulk specific gravity of approximately 8.
It has also been suggested to provide structural damping using polymeric viscoelastic spheres. Viscoelastic materials absorb energy when they are deformed. See, J. R. House, "Damping Hollow Tubular Structures with Lightweight Viscoelastic Spheres," Proceedings of the ACS Division of Polymeric Materials: Science and Engineering, 60, Spring Meeting 1989, Dallas, Tex. In this article, House proposes the use of spheres molded from an epoxy resin.
The prior art structural damping techniques thus involve granular material having either significant mass relative to the host structure such as sand or lead shot, or high intrinsic damping characteristics such as viscoelastic spheres. While viscoelasticity and mass are useful, neither are essential as the present invention will make clear.